Evolution of Oxygen Vacancy Sites in Ceria-Based High-Entropy Oxides and Their Role in N2 Activation

In this work, a relatively new class of materials, rare earth (RE) based high entropy oxides (HEO) are discussed in terms of the evolution of the oxygen vacant sites (Ov) content in their structure as the composition changes from binary to HEO using both experimental and computational tools; the composition of HEO under focus is the CeLaPrSmGdO due to the importance of ceria-related (fluorite) materials to catalysis. To unveil key features of quinary HEO structure, ceria-based binary CePrO and CeLaO compositions as well as SiO2, the latter as representative nonreducible oxide, were used and compared as supports for Ru (6 wt % loading). The role of the Ov in the HEO is highlighted for the ammonia production with particular emphasis on the N2 dissociation step (N2(ads) → Nads) over a HEO; the latter step is considered the rate controlling one in the ammonia production. Density functional theory (DFT) calculations and 18O2 transient isotopic experiments were used to probe the energy of formation, the population, and the easiness of formation for the Ov at 650 and 800 °C, whereas Synchrotron EXAFS, Raman, EPR, and XPS probed the Ce–O chemical environment at different length scales. In particular, it was found that the particular HEO composition eases the Ov formation in bulk, in medium (Raman), and in short (localized) order (EPR); more Ov population was found on the surface of the HEO compared to the binary reference oxide (CePrO). Additionally, HEO gives rise to smaller and less sharp faceted Ru particles, yet in stronger interaction with the HEO support and abundance of Ru–O–Ce entities (Raman and XPS). Ammonia production reaction at 400 °C and in the 10–50 bar range was performed over Ru/HEO, Ru/CePrO, Ru/CeLaO, and Ru/SiO2 catalysts; the Ru/HEO had superior performance at 10 bar compared to the rest of catalysts. The best performing Ru/HEO catalyst was activated under different temperatures (650 vs 800 °C) so to adjust the Ov population with the lower temperature maintaining better performance for the catalyst. DFT calculations showed that the HEO active site for N adsorption involves the Ov site adjacent to the adsorption event.

Raman.Raman spectroscopy was performed to complement powder XRD and probe the oxygen sublattice and induced structural defects.Raman spectroscopy (Horiba JobinYvon) instrument, green laser (λ = 633 nm) and 50 × objective lens was used.

XPS.
The as-synthesized catalysts were analyzed using XPS in order to assess the effectiveness of the in situ Ru reduction applied during the synthesis.The elemental concentrations and chemical states on the surface were assessed through X-ray photoelectron spectroscopy (XPS) using a ThermoScientific™ ESCALAB™ QXi Spectrometer equipped with a monochromated Al Kα X-ray source with a photon energy of 1486.6 eV.

HRTEM.
To analyze the samples, Titan 80-300 ST transmission electron microscope (TEM) (Thermo Fisher Scientific Inc.) was utilized to carry out TEM analysis.The analysis was performed by operating the microscope at the accelerating voltage of 300 kV.The microscope was set to bright-field (BF) TEM imaging so to acquire images of the samples both at low magnification (LM) and high magnification (HM); this allowed the investigation of the morphology and structure of the nanoparticles (NPs), respectively.Furthermore, the structural analysis of the NPs was conducted using the selected area electron diffraction (SAED) technique.The d-spacings measured from the acquired SAED patterns were compared with the ones obtained through XRD.To determine the distributions of each element in the samples at the nanometer scale, we utilized electron energy loss spectroscopy (EELS) coupled with scanning transmission electron microscopy (STEM).
The utilization of EELS generated Ru maps provided an effective means to determine the size of the Ru NPs, compared to using the concentrated Bright-field TEM image, which can be influenced by differences in material thickness.

H2 Chemisorption (H2-TPD).
The H2-TPD technique was used to study the dispersion of the supported Ru catalysts.Calcination of the catalyst at 650 o C/4 h (static air, furnace) was the first step.Then, 0.1 g of sample was loaded in the U-tube microreactor, and the temperature was increased under He gas flow up to 650 o C. At these conditions reduction of the sample took place in hydrogen gas flow (1 bar) at 650 o C/2 h, followed by He purge at 650 o C until the H2-TCD signal was stabilized at its background value.The catalyst was cooled down to 30 o C in He flow; then 30-min of exposure to a 0.5 vol.%H2/He adsorption gas followed.In order to limit the extent of the H-spillover effect use of lower or higher adsorption times in H2/He resulted in very similar chemisorption amounts (within experimental error).After H2 chemisorption, the sample was purged in He flow for 10 min and its temperature was then increased to 700 o C (β = 30 o C min −1 , H2-TPD).The H2 signal (m/z = 2) was continuously monitored with online thermal conductivity detector (TCD) and converted into concentration (mol%) using a certified gas mixture (0.95 vol% H2/He).
EPR.The electron paramagnetic resonance (EPR) studies were conducted with a Bruker ELEXSYS E500 spectrometer operating at the X band, and using a continuous wave (CW) setup.The spectrometer featured a superhigh Q (ER 4122 SHQ) resonator.In this study the EPR spectra were collected at two temperatures (298 K and 100 K).The microwave frequency of 9.42 GHz with 20 dB microwave attenuation, 5 G modulation amplitude, and 100 kHz modulation frequency were used.The Bruker Xenon software (Bruker BioSpin, Rheinstetten, Germany) was employed for data collection.Table S3.Areas of the O1s deconvoluted peaks.
Table S4: Ru dispersion and particle size as obtained from the H2 chemisorption studies.Table S5: FFT analysis of the Ru/HEO catalyst.
Table S6: FFT analysis of the Ru/HEO catalyst.

Figure S2 :
Figure S2: SEM microphotographs obtained over the Ru catalysts of this study.

Table S2 :
SEM-EDS elemental composition of the catalysts of the present study.

Table S1 :
Textural properties of the catalysts in this study.